1. Field of the Invention
This invention relates to new and useful improvements in processes for the preparation of sand cores and molds for foundry application and more particularly to improved processes using acid-curable resin binders for sand in which the resin is cured by in situ formation of sulfuric acid and in which the molds or cores have improved tensile strength and hardness and characterized by substantial freedom from formation of resinous coatings on the patterns for the cores or molds.
2. Description of the Prior Art
In the foundry industry, sand is coated with resin binders and formed into molds and cores for the production of precision castings. A wide variety of techniques have been developed for the manufacture of sand cores and molds. These involve the hot box technique for mold and core formation; the shell method; the "No-Bake", and the cold-box technique.
In the hot box and shell methods, sand molds and cores are formed by heating a mixture of sand a thermo setting resin at a temperature of about 300.degree.-600.degree. F. in contact with patterns which produce the desired shape for the mold or core. The resin is polymerized and a core or mold is formed. Procedures of this type are described in Dunn et al. U.S. Pat. No. 3,059,297 and Brown et al. U.S. Pat. No. 3,020,609.
A particular disadvantage of the hot box and shell methods is the necessity for heating the pattern boxes to 300.degree.-600.degree. F. to polymerize and cure the resin binder. This involves considerable expense and is generally a high cost technique.
The cold box techniques for core and mold formation involve the use of sand mixed or coated with resins which may be cured at room temperature by acid or base catalysis. Acid or base catalysts have been used in liquid, solid or gaseous form. Typical cold box processes are shown in Blaies U.S. Pat. No. 3,008,205; Dunn et al. U.S. Pat. No. 3,059,297; Peters et al. U.S. Pat. No. 3,108,340; Kottke et al. U.S. Pat. No. 3,145,438; Brown et al. U.S. Pat. No. 3,184,814; Robins U.S. Pat. No. 3,639,654; Australian Pat. No. 453,160 and British Pat. No. 1,225,984. Many of these processes involve the use of sulfur-containing acid catlayst such as benzene sulfonic acid, toluene sulfonic acid and the like.
Recently, a process has been developed for room temperature polymerization of condensation resins in which the acid-curing agent is generated in situ in the resin or on a sand-resin mix. It had previously been suggested in U.S. Pat. No. 3,145,438 to inject SO.sub.3 in a form of a gas into a mixture of sand and resin to cure the resin at room temperature. It was found, however, that this process causes an instantaneous curing of the resin in the region subjected to treatment by SO.sub.3 which impedes the diffusion of this gas to other parts of the resin, particularly the central parts of the mixture. Subsequently, a method was developed which avoided this difficulty. In Richard U.S. Pat. No. 3,879,339, it is disclosed that sand may be coated with a suitable oxidizing agent, such as an organic peroxide, and coated with the resin to be used in binding the sand into the form of a core or mold. The sand-resin mixture is then formed into suitable shape and treated with gaseous SO.sub.2. The SO.sub.2 is oxidized, in situ, to SO.sub.3 and converted to sulfur-containing acid by water present in the mixture. The sulfur-containing acid which is generated in situ causes a rapid and uniform polymerization of the resin at room temperature. This process has proved successful commercially and is applicable to phenolic resins, furan resins, and urea-formaldehyde resins, as well as mixtures and copolymers thereof.
In the cold box method of Richard U.S. Pat. No. 3,879,339 it is desirable to use resin compositions which are extended by admixture with furfuryl alcohol. It has been common practice to extend phenolic resins, furan resins, and urea-formaldehyde resins by admixture with furfuryl alcohol. One serious problem, however, has arisen in connection with the use of furfuryl alcohol extended resins. In the cold box method of Richard U.S. Pat. No. 3,879,339 and in some other cold-box processes it has been found that when furfuryl alcohol extended resins are used there is an undesirable migration of furfuryl alcohol to the patterns used to form the molds or cores. This results in sticking of the resins to the patterns and makes removal of the molds or cores from the patterns quite difficult. Also, there is a tendency to build up a substantial layer of furfuryl alcohol resin condensate on the patterns. As a result, there has been a substantial need for resin compositions which may include extenders or modifiers of the furfuryl alcohol type which do not exhibit this undesirable migration to the pattern or form.
As noted above, the use of furfuryl alcohol-modified resins is known in the resin art and known in the art of foundry core manufacture, particularly as illustrated by the following references.
Treat et al. U.S. Pat. No. 2,999,829 discloses the copolymerization of furfuryl alcohol and maleic anhydride. This resin composition is disclosed to be useful in the preparation of sand base foundry shell molds.
Brown U.S. Pat. No. 3,216,075 discloses the preparation of foundry cores and molds using snad bonded by a resin consisting of a furfuryl alcohol-formaldehyde composition containing an acid catalyst and boron oxides or boron acids.
Bornstein U.S. Pat. No. 3,244,648 discloses resins of the phenol-furfuryl type.
Case et al. U.S. Pat. No. 3,212,650 discloses resins based on phenol and furfuryl alcohol modified with formaldehyde and treated with an acid catalyst.
Cleek et al. U.S. Pat. No. 3,538,035 discloses a mixture of furfuryl alcohol with a urea-formaldehyde pre-condensate as a preliminary step in the preparation of foundry cores or molds.
Adkins et al. U.S. Pat. No. 3,725,333 discloses the preparation of foundry cores or molds from sand and a curing agent and a phenol-formaldehyde resin binder which has been modified with furfural or furfuryl alcohol.
Johnson U.S. Pat. No. 3,755,229 discloses a foundry core composition comprising sand and a binder comprising phenolic resole resin, furfuryl alcohol and a stannic chloride or stannous chloride catalyst. The furfuryl alcohol is mixed with the preformed phenolic condensate.
Kawai et al. U.S. Pat. No. 3,893,964 discloses an acid-hardening binder for foundry sand which is a resinous composition comprising furfuryl alcohol and a urea-formaldehyde-glyoxal condensate.
The above-listed references are illustrative of the use of furfuryl alcohol as a modifier or extender for various condensation-type resins.
The various Patentees, however, did not consider the problem of migration of furfuryl alcohol and its condensation products to the forms or patterns during the curing of the sand-resin mixture. These inventors did not consider this problem and did not arrive at a solution for it.